This invention relates to the production of nuclear fuel bodies and more particularly to the production of nuclear fuel bodies which are molded from a mixture of pyrolytic carbon-coated fuel particles, graphite, and a suitable binder, such as pitch or the like.
Nuclear fuel bodies for nuclear reactors, for example high-temperature gas-cooled reactors, have a relatively large core area dnd thus require relatively large variations of the fissile and fertile loadings of fuel bodies to be disposed at different regions within the core. To economically produce fuel for reactors of this type, it is considered necessary to be able to produce fuel bodies of uniform dimensions which, in some simple fashion, will accommodate the relatively large variations of the fuel loading requirements.
One way in which this object has been accomplished has been by employing an inert filler or "shim", usually in the form of particles of graphite of a size generally approximating the size of the fuel particles, and by mixing the particulate graphite in varying amounts with the coated nuclear fuel particles to produce the fuel loading desired for a particular nuclear fuel body. The use of graphite particles in fuel bodies is considered to insure high-temperature and irradiation stability and to provide improved thermal conductivity. Moreover, it does not adversely affect the neutron economics of the core because graphite actually adds to the inventory of moderator in a reactor wherein the fuel elements themselves are made from a carbon moderating material, for example, blocks of graphite.
It has been found that two of the important properties of such shim graphite, for the purpose of insuring reproducible and uniform fuel body fabrication, are its strength and its apparent density or degree of porosity. The particulate graphite should have adequate strength to minimize compaction under the pressures which are normally necessary in molding green fuel bodies, and it is desired that particulate graphite be available having uniformly good structural strength and low porosity.